Best Price PSU for this build

650w would be plenty for that build. Infact 500 would probably be enough. The only reason you would need higher is if you wanted to SLI that 560/570 in which case you would be looking at 750-850w+

As far as 500-650w goes:

£50 Corsair 600W CX http://www.ebuyer.com/product/269259
£65 Antec TruePower 550W Modular PSU http://www.ebuyer.com/product/159918
£66 OCZ ModXStream Pro 600W http://www.ebuyer.com/product/174182
£69 Corsair TX 650W V2 http://www.ebuyer.com/product/257232

Apologies if your in the US and the links and prices aren't helpful, but the PSU brands and models still apply  

500W would definitely be enough for a system with one GTX 560/570.
Another unit worth considering is the Antec HCG-520.

SLI GTX 560s can easily be done on a good 650W PSU, try to choose one that has four PCIe connectors, unless you are happy to use peripheral to PCIe adapters. There aren't that many 650W PSUs with four PCIe connectors around at good prices at the moment tho.

XFX sell quite a few good PSUs at good prices.